The effect of steam on the acid strength of H3PW12O40 (HPW)/SiO2 . nH(2)O was determined by the Hammett indicators. When the steam content in N-2 was 1.6%, the acidity (H-0 > -13.7) of HPW/SiO2 . nH(2)O could be kept for over 10 h at 300 degreesC, but for only 2 h when steam was lacking. When it was used as a catalyst for skeletal isomerization of n-butane to isobutane at 300 degreesC, the HPW/SiO2 . nH(2)O showed stability in the presence of steam (1.6% in feed) better than that without steam, due to a suppression of the loss of crystalline water. (C) 2003 Elsevier B.V. All rights reserved.